Hot extrusion of metal powder is a feasible technique to consolidate metal powders to fully dense products. Extrusion offers distinct advantages for long products like bars, rods, tubes and profiles not only because of high yields but also because "difficult to make" metals can be produced directly to a certain shape.
The specific deformation pattern characterized by hot extrusion also tends to break up surface films, such as oxides and other impurities on the powders, thereby giving a much better quality to the finished product.
Refractory metals, such as tantalum and niobium and their alloys can be extruded. However, one of the characteristics of these metals is that their raw material cost is extremely high. This makes it very important to optimize the yield in each operation. In the case of extrusion, it is important to minimize the losses caused by imperfections and especially also to minimize losses in the front and back ends of the extrusion body. As described below, a normal appearance of an extruded billet from a solid material is shown in FIG. 2 and the same for a powder billet is shown in FIG. 4. The total product yield after extrusion is very seldom over 85% for a powder capsule and seldom over 90% for a solid material. The reasons for the lower yield for the powder billet is that a front plate is drawn over the powder at extrusion and a back end plate is sucked into the powder at the other end.
Tantalum powder can be made from tantalum solid metal which is hydrided, crushed, and dehydrided to powder form or produced from potassium tantalum fluoride by sodium reduction or by other means. This irregular powder is then pressed to a small bar which is sintered in vacuum at a high temperature to form a rod which is then processed in several steps down to finer dimensions, for example, through cold rolling. Alternatively, tantalum is electron beam melted or arc cast melted to produce ingots which are processed into rod, sheet or tubing, for example, by forging, swaging, rolling, etc.
In powder form, tantalum picks up oxygen at the hydriding, dehydriding and melting stages, Subsequent sintering in vacuum is also a refining step where not only oxygen but also other impurities are removed as a result of high temperature and a high vacuum. Tantalum forms extremely stable oxides. High purity, low oxygen tantalum powder which is exposed to air, even at room temperature, can be a safety risk; the finer the powder is, the higher the risk.
The conventional way of processing tantalum is a very costly process. Not only the many process steps but also production of the rod, sheet or tubing provide low product yield which affect the final production costs dramatically.
As described in copending patent application Ser. No. 08/146,788, filed Nov. 2, 1993, now U.S. Pat. No. 5,445,787, it has also been known to form an extruded product of a powder charge of tantalum or niobium in a process in which a charge of the powdered metal is subjected to a sequence of cold isostatic pressing steps and heating steps prior to extruding the compressed charge. Typically, the powdered metal is first compressed into a green compact or billet which is then placed in a capsule of carbon steel and heated to an annealing temperature. Thereafter, the capsule is again cold isostatically pressed and then heated and extruded into an extruded product.
However, even in this case, it is extremely difficult to increase the yield from the powder in the billet to the extruded product to more than 85%. Even if this is a normal yield and far higher than the yield obtained in a conventional process to produce tantalum, it is of important economical value to reach as high a yield as possible.
Furthermore, during the final cold isostatic pressing of the metal capsule, there is always a risk of a leakage when a water emulsion subjects the powder billet to a pressure of up to 500 MPa (75,000 psi). In such a case, the water represents a risk during the following heating of the billet. That is to say, the water which has penetrated the billet can transform into steam during heating and create an explosive situation. There are techniques to detect such water but if the water-extrusion penetrates the tantalum powder, the powder is in principle destroyed and must be scrapped. A normal yield loss in cold isostatic pressing is between 1-5%, thus, further decreasing the efficiency of the process.
One way to ensure that no water exists during the final heating before extrusion is to vacuum pump the billet, preferably under combined vacuum and heat.
Accordingly, it is an object of the invention to be able to produce products made of tantalum and/or niobium at a relatively low cost and with a high density.
It is another object of the invention to provide a relatively simple technique for extruding tantalum and niobium powders into extruded products of high density.
It is another object of the invention to provide an improved method of forming high density tantalum and niobium products.
Briefly, the invention provides a process of forming extruded products of tantalum and/or niobium.
In accordance with the process, a charge of powdered metal selected from the group consisting of tantalum and niobium is cold isostatically pressed to a density sufficient to form a green compact or billet with a sufficient strength to be handled. Preferably, the compressed billet of tantalum is made by the known so-called wet bag process. The cold isostatic pressure is chosen so that the obtained density is between 70-85% of the theoretical density. The compressed billet is then released from the wet bag and placed in a container or capsule which is then sealed with end caps by welding. The material of this container could consist of carbon steel or of tantalum or niobium, i.e. the same material as the compressed powder billet.
The carbon steel used, could typically be of low carbon content to avoid segregation during the following heating. For example, such carbon content could be less than 0.005% and typically in the range of 0.002- 0.003%. With such low carbon content, carbon pick up is avoided, for example, into the tantalum, which is very sensitive even for small amounts of impurities like carbon.
In the process described in application Ser. No. 08/146,788, if a wet bag operation is made and the compressed powder billet is placed in a metal container, a second heating operation is necessary before the second cold isostatic pressing reaches a sufficiently good extrusion result. The commonly accepted reason is that in order to extrude thin-walled capsules, the density of the powder must be approximately a minimum of 80% of the theoretical density to avoid wrinkling of the capsule thereby causing imperfections.
In accordance with the invention, the wet bag compressed billet is placed in a metal container with narrow tolerances between the billet and the container. No annealing is made but other operations can be done, for example, in the case of tantalum, evacuation of the container or dehydriding of the tantalum powder can be performed while subjecting the tantalum in the container to a vacuum at moderate temperature (600.degree. to 1000.degree. C.).
The metal container is then placed in a second metal container with an annular gap between the two containers. This gap is filled with a carbon steel powder or another type of metal produced with a spherical shape which gives a high filling density after vibration, i.e. approximately 70%, and with a yield strength (flow stress) substantially lower than the enclosed tantalum or niobium at the extrusion temperature.
It is important before filling to place the inner metal container concentrically in the outer container in order to ensure a satisfactory extrusion result. Accordingly, spacers may be provided to maintain an annular gap between the containers.
This double container is then sealed, for example, using end caps, and cold isostatically pressed in order to avoid segregation of the metal powder in the cap between the containers during the subsequent handling before extrusion. The isostatic pressing may be performed at a pressure of at least 200 mpa for this purpose. As the tantalum or niobium powder is already cold isostatically pressed once, the density of this material will not be affected. The hardness of the carbon steel is (in the atomized condition) so high that the density of this surrounding material, also after cold isostatic pressing, is increased very little to just slightly over 70%.
Thereafter, the compressed double container is heated and extruded to form an extruded product, for example, of bar shape. Typically, the front and rear ends of the bar are primarily made of the material of the end caps which serve to seal the containers and the compacted powder in the outer container. Hence, the front and rear ends of the extruded product can be cut off and removed leaving a bar which is primarily made of tantalum or niobium, as the case may be. Typically, the extruded rod at this point has a yield of from 93% to 96% of the beginning powder.